3.4. Production of mixed acids by mixed culture with isolated
microbial strains
To further examine the effect of mixed culture conditions on
mixed acid production, we constructed a microbial community and
applied it to the fermentation process to produce mixed acids from
non-pretreated red algae (Fig. 1). Direct production of mixed acids
without pretreatment was observed in all fermentation reactions.
After 15 days of fermentation in a 5-L bioreactor (working volume,
2.4 L), the highest yield of mixed acids from the defined mixed
culture was about 35% (29.17 g/L), which was comparable to that of
the enrichment culture. In the defined mixed culture, mixed acids
were consistently produced for up to 60 days. The maximal yield
reached a much higher level in the defined mixed culture (50.90%)
than in the undefined mixed culture (40.72%). Thus, the defined
mixed culture identified here can be used to enhance mixed acids
conversion from raw G. amansii in anaerobic cultures. Several
studies have investigated mixed acid production from various
substrates, such as sludge, waste and office copier paper, reporting
a mixed acid concentration of approximately 14e26 g/L [12e14]. In
fixed-bed fermentation of hydrolyzed brown seaweed, the maximum acid concentration reported was approximately 6.0 g/L,
and the main products detected were acetic acid and propionic acid
[10]. Thus, at more than 29 g/L, the concentration produced from
anaerobic batch culture of G. amansii reported here exceeds the
high end of values reported for other substrates.